Concept for a Directional Color Filter

Schematic shows two different ways that white light interacts with a newly developed device, a directional color filter ruled with grooves that are not uniformly spaced. (Source: NIST)

Imagine a minia­ture device that suffuses each room in your house with a different hue of the rainbow – purple for the living room, perhaps, blue for the bedroom, green for the kitchen. A team led by scientists at the National Institute of Standards and Tech­nology NIST has, for the first time, developed nanoscale devices that divide incident white light into its component colors based on the direction of illu­mination, or directs these colors to a prede­termined set of output angles.

Viewed from afar, the device, referred to as a direc­tional color filter, resembles a dif­fraction grating, a flat metal surface containing parallel grooves or slits that split light into different colors. However, unlike a grating, the nano­meter-scale grooves etched into the opaque metal film are not periodic, not equally spaced. They are either a set of grooved lines or concen­tric circles that vary in spacing, much smaller than the wave­length of visible light. These proper­ties shrink the size of the filter and allow it to perform many more functions than a grating can.

For instance, the device’s nonuniform, or aperiodic, grid can be tailored to send a parti­cular wave­length of light to any desired location. The filter has several promising appli­cations, including gene­rating closely spaced red, green and blue color pixels for displays, har­vesting solar energy, sensing the direction of incoming light and measuring the thickness of ultrathin coatings placed atop the filter.

In addition to selec­tively filtering incoming white light based on the location of the source, the filter can also operate in a second way. By measuring the spectrum of colors passing through a filter custom-designed to deflect specific wave­lengths of light at specific angles, researchers can pinpoint the location of an unknown source of light striking the device. This could be critical to determine if that source, for instance, is a laser aimed at an aircraft.

“Our direc­tional filter, with its aperio­dic archi­tecture, can function in many ways that are funda­mentally not achievable with a device such as a grating, which has a periodic structure,” said NIST physicist Amit Agrawal. “With this custom-designed device, we are able to mani­pulate multiple wave­lengths of light simul­taneously.” (Source: NIST)

Reference: M. S. Davis et al.: Aperiodic nanoplasmonic devices for directional colour filtering and sensing, Nat. Commun. 8, 1347 (2017); DOI: 10.1038/s41467-017-01268-y

Link: Center for Nanoscale Science and Technology, National Inst. of Standards and Technology NIST, Gaithersburg, USA

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